Techniques for programming of select gates in NAND memory

ABSTRACT

In a non-volatile memory formed according to a NAND-type architecture that has, on one or both ends of the NAND strings, multiple select gates including some with programmable threshold voltages, a structure and corresponding for efficiently programming of such select gates. On the drain side, the end most of multiple drain select transistors is individually controllable and used for biasing purposes while one or more other drain side select gates are collectively programmed to set adjust their threshold voltage. Independently, on the source side, the end most of multiple source select transistors is individually controllable and used for biasing purposes while other source side select gates are collectively programmed to set adjust their threshold voltage.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.15/062,987, filed Mar. 7, 2016, issued as U.S. Pat. No. 9,659,656,entitled “Techniques for Programming of Select Gates in NAND Memory,”which is a continuation of U.S. application Ser. No. 14/464,122, filedAug. 20, 2014, issued as U.S. Pat. No. 9,305,648, entitled “Techniquesfor Programming of Select Gates in NAND Memory,” the contents of whichare incorporated herein by reference in their entirety.

BACKGROUND

This application relates to the operation of re-programmablenon-volatile memory systems such as semiconductor flash memory thatrecord data using charge stored in charge storage elements of memorycells.

Solid-state memory capable of nonvolatile storage of charge,particularly in the form of EEPROM and flash EEPROM packaged as a smallform factor card, has recently become the storage of choice in a varietyof mobile and handheld devices, notably information appliances andconsumer electronics products. Unlike RAM (random access memory) that isalso solid-state memory, flash memory is non-volatile, and retains itsstored data even after power is turned off. Also, unlike ROM (read onlymemory), flash memory is rewritable similar to a disk storage device. Inspite of the higher cost, flash memory is increasingly being used inmass storage applications.

Flash EEPROM is similar to EEPROM (electrically erasable andprogrammable read-only memory) in that it is a non-volatile memory thatcan be erased and have new data written or “programmed” into theirmemory cells. Both utilize a floating (unconnected) conductive gate, ina field effect transistor structure, positioned over a channel region ina semiconductor substrate, between source and drain regions. A controlgate is then provided over the floating gate. The threshold voltagecharacteristic of the transistor is controlled by the amount of chargethat is retained on the floating gate. That is, for a given level ofcharge on the floating gate, there is a corresponding voltage(threshold) that must be applied to the control gate before thetransistor is turned “on” to permit conduction between its source anddrain regions. Flash memory such as Flash EEPROM allows entire blocks ofmemory cells to be erased at the same time.

The floating gate can hold a range of charges and therefore can beprogrammed to any threshold voltage level within a threshold voltagewindow. The size of the threshold voltage window is delimited by theminimum and maximum threshold levels of the device, which in turncorrespond to the range of the charges that can be programmed onto thefloating gate. The threshold window generally depends on the memorydevice's characteristics, operating conditions and history. Eachdistinct, resolvable threshold voltage level range within the windowmay, in principle, be used to designate a definite memory state of thecell.

In order to improve read and program performance, multiple chargestorage elements or memory transistors in an array are read orprogrammed in parallel. Thus, a “page” of memory elements are read orprogrammed together. In existing memory architectures, a row typicallycontains several interleaved pages or it may constitute one page. Allmemory elements of a page are read or programmed together.

Nonvolatile memory devices are also manufactured from memory cells witha dielectric layer for storing charge. Instead of the conductivefloating gate elements described earlier, a dielectric layer is used.Such memory devices utilizing dielectric storage element have beendescribed by Eitan et al., “NROM: A Novel Localized Trapping, 2-BitNonvolatile Memory Cell,” IEEE Electron Device Letters, vol. 21, no. 11,November 2000, pp. 543-545. An ONO dielectric layer extends across thechannel between source and drain diffusions. The charge for one data bitis localized in the dielectric layer adjacent to the drain, and thecharge for the other data bit is localized in the dielectric layeradjacent to the source. For example, U.S. Pat. Nos. 5,768,192 and6,011,725 disclose a nonvolatile memory cell having a trappingdielectric sandwiched between two silicon dioxide layers. Multi-statedata storage is implemented by separately reading the binary states ofthe spatially separated charge storage regions within the dielectric.

SUMMARY OF THE INVENTION

In a first set of aspects, a non-volatile semiconductor memory devicehas a memory array structure that includes a first bit line, a sourceline, and a first NAND string. The first NAND string includes: aplurality of non-volatile memory cells connected in series between thesource line and the first bit line; one or more serially connectedsource select transistors through which the memory cells are connectedto the source line; and a plurality of serially connected drain selecttransistors through which the memory cells are connected to the firstbit line, wherein the serially connected drain select transistorsinclude a first drain select transistor and one or more second drainselect transistors, wherein the second drain select transistors have aprogrammable threshold level and are connected to the first bit linethrough the first select drain transistor. The memory array structurealso includes read/write circuitry connectable to the first bit line,the source line, and the control gates of the memory cells, and thecontrol gates of the source and drain select transistors. The seconddrain select transistors are programmed by applying a sequence of one ormore common programming pulses concurrently to the control gates of thesecond drain select transistors while the first bit line is initiallyset to a program enable level, the source line is set high, the controlgates of the memory cells are set at a pass level, one or more of thesource select transistors are turned off, and the first drain selecttransistor is turned on.

In other aspects, a non-volatile semiconductor memory device has amemory array structure that includes a first bit line, a source line,and a first NAND string. The first NAND string includes: a plurality ofnon-volatile memory cells connected in series between the source lineand the first bit line; one or more serially connected drain selecttransistors through which the memory cells are connected to the firstbit line; and a plurality of serially connected source selecttransistors through which the memory cells are connected to the sourceline, wherein the serially connected source select transistors include asource drain select transistor and one or more second source selecttransistors, wherein the second source select transistors have aprogrammable threshold level and are connected to the source linethrough the first source drain transistor. The memory array structurealso includes read/write circuitry connectable to the first bit line,the source line, and the control gates of the memory cells, and thecontrol gates of the source and drain select transistors. The secondsource select transistors are programmed by applying a sequence of oneor more common programming pulses concurrently to the control gates ofthe second source select transistors while the first bit line isinitially set to a program enable level, the source line is set high,the control gates of the memory cells are set at a pass level, the firstsource select transistor is turned off, and the drain select transistorsare turned on.

Various aspects, advantages, features and embodiments of the presentinvention are included in the following description of exemplaryexamples thereof, which description should be taken in conjunction withthe accompanying drawings. All patents, patent applications, articles,other publications, documents and things referenced herein are herebyincorporated herein by this reference in their entirety for allpurposes. To the extent of any inconsistency or conflict in thedefinition or use of terms between any of the incorporated publications,documents or things and the present application, those of the presentapplication shall prevail.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates schematically the main hardware components of amemory system suitable for implementing the present invention.

FIG. 2 illustrates schematically a non-volatile memory cell.

FIG. 3 illustrates the relation between the source-drain current I_(D)and the control gate voltage V_(CG) for four different charges Q1-Q4that the floating gate may be selectively storing at any one time atfixed drain voltage.

FIG. 4 illustrates schematically a string of memory cells organized intoa NAND string.

FIG. 5 illustrates an example of a NAND array 210 of memory cells,constituted from NAND strings 50 such as that shown in FIG. 4.

FIG. 6 illustrates a page of memory cells, organized in the NANDconfiguration, being sensed or programmed in parallel.

FIGS. 7A-7C illustrate an example of programming a population of memorycells.

FIG. 8 shows an example of a physical structure of a 3-D NAND string.

FIGS. 9-12 look at a particular monolithic three dimensional (3D) memoryarray of the NAND type (more specifically of the “BiCS” type).

FIG. 13 shows a typical set of bias waveforms for programming selectedmemory cells in a NAND type memory.

FIGS. 14A and 14B schematically illustrate the effects on theprogramming of a string of memory cells when the selected gatesrespectively have positive and negative threshold voltages.

FIG. 15 shows an exemplary set of bias voltages for sensing operation.

FIG. 16 illustrates the mechanism whereby an unselected NAND stringhaving select gates with negative thresholds can affect a sensingoperation in the 3D context.

FIG. 17 is a two dimensional projection of a block of a typical BiCStype structure, showing one NAND string sharing a common bit line foreach of four finger, to illustrate some of the structure on how theselect gates and memory cells are arranged.

FIG. 18 illustrates an exemplary topology for the control gateconnections of drain side select gates and bias levels for theirprogramming.

FIG. 19 represents the resultant channel boosting from the biasing shownin FIG. 18.

FIG. 20 illustrates an exemplary topology for the control gateconnections of source side select gates and bias levels for theirprogramming.

FIG. 21 represents the resultant channel boosting from the biasing shownin FIG. 20.

DETAILED DESCRIPTION

Memory System

FIG. 1 illustrates schematically the main hardware components of amemory system suitable for implementing the present invention. Thememory system 90 typically operates with a host 80 through a hostinterface. The memory system may be in the form of a removable memorysuch as a memory card, or may be in the form of an embedded memorysystem. The memory system 90 includes a memory 102 whose operations arecontrolled by a controller 100. The memory 102 comprises one or morearray of non-volatile memory cells distributed over one or moreintegrated circuit chip. The controller 100 may include interfacecircuits 110, a processor 120, ROM (read-only-memory) 122, RAM (randomaccess memory) 130, programmable nonvolatile memory 124, and additionalcomponents. The controller is typically formed as an ASIC (applicationspecific integrated circuit) and the components included in such an ASICgenerally depend on the particular application.

With respect to the memory section 102, semiconductor memory devicesinclude volatile memory devices, such as dynamic random access memory(“DRAM”) or static random access memory (“SRAM”) devices, non-volatilememory devices, such as resistive random access memory (“ReRAM”),electrically erasable programmable read only memory (“EEPROM”), flashmemory (which can also be considered a subset of EEPROM), ferroelectricrandom access memory (“FRAM”), and magnetoresistive random access memory(“MRAM”), and other semiconductor elements capable of storinginformation. Each type of memory device may have differentconfigurations. For example, flash memory devices may be configured in aNAND or a NOR configuration.

The memory devices can be formed from passive and/or active elements, inany combinations. By way of non-limiting example, passive semiconductormemory elements include ReRAM device elements, which in some embodimentsinclude a resistivity switching storage element, such as an anti-fuse,phase change material, etc., and optionally a steering element, such asa diode, etc. Further by way of non-limiting example, activesemiconductor memory elements include EEPROM and flash memory deviceelements, which in some embodiments include elements containing a chargestorage region, such as a floating gate, conductive nanoparticles, or acharge storage dielectric material.

Multiple memory elements may be configured so that they are connected inseries or so that each element is individually accessible. By way ofnon-limiting example, flash memory devices in a NAND configuration (NANDmemory) typically contain memory elements connected in series. A NANDmemory array may be configured so that the array is composed of multiplestrings of memory in which a string is composed of multiple memoryelements sharing a single bit line and accessed as a group.Alternatively, memory elements may be configured so that each element isindividually accessible, e.g., a NOR memory array. NAND and NOR memoryconfigurations are exemplary, and memory elements may be otherwiseconfigured.

The semiconductor memory elements located within and/or over a substratemay be arranged in two or three dimensions, such as a two dimensionalmemory structure or a three dimensional memory structure.

In a two dimensional memory structure, the semiconductor memory elementsare arranged in a single plane or a single memory device level.Typically, in a two dimensional memory structure, memory elements arearranged in a plane (e.g., in an x-z direction plane) which extendssubstantially parallel to a major surface of a substrate that supportsthe memory elements. The substrate may be a wafer over or in which thelayer of the memory elements are formed or it may be a carrier substratewhich is attached to the memory elements after they are formed. As anon-limiting example, the substrate may include a semiconductor such assilicon.

The memory elements may be arranged in the single memory device level inan ordered array, such as in a plurality of rows and/or columns.However, the memory elements may be arrayed in non-regular ornon-orthogonal configurations. The memory elements may each have two ormore electrodes or contact lines, such as bit lines and word lines.

A three dimensional memory array is arranged so that memory elementsoccupy multiple planes or multiple memory device levels, thereby forminga structure in three dimensions (i.e., in the x, y and z directions,where the y direction is substantially perpendicular and the x and zdirections are substantially parallel to the major surface of thesubstrate).

As a non-limiting example, a three dimensional memory structure may bevertically arranged as a stack of multiple two dimensional memory devicelevels. As another non-limiting example, a three dimensional memoryarray may be arranged as multiple vertical columns (e.g., columnsextending substantially perpendicular to the major surface of thesubstrate, i.e., in the y direction) with each column having multiplememory elements in each column. The columns may be arranged in a twodimensional configuration, e.g., in an x-z plane, resulting in a threedimensional arrangement of memory elements with elements on multiplevertically stacked memory planes. Other configurations of memoryelements in three dimensions can also constitute a three dimensionalmemory array.

By way of non-limiting example, in a three dimensional NAND memoryarray, the memory elements may be coupled together to form a NAND stringwithin a single horizontal (e.g., x-z) memory device levels.Alternatively, the memory elements may be coupled together to form avertical NAND string that traverses across multiple horizontal memorydevice levels. Other three dimensional configurations can be envisionedwherein some NAND strings contain memory elements in a single memorylevel while other strings contain memory elements which span throughmultiple memory levels. Three dimensional memory arrays may also bedesigned in a NOR configuration and in a ReRAM configuration.

Typically, in a monolithic three dimensional memory array, one or morememory device levels are formed above a single substrate. Optionally,the monolithic three dimensional memory array may also have one or morememory layers at least partially within the single substrate. As anon-limiting example, the substrate may include a semiconductor such assilicon. In a monolithic three dimensional array, the layersconstituting each memory device level of the array are typically formedon the layers of the underlying memory device levels of the array.However, layers of adjacent memory device levels of a monolithic threedimensional memory array may be shared or have intervening layersbetween memory device levels.

Then again, two dimensional arrays may be formed separately and thenpackaged together to form a non-monolithic memory device having multiplelayers of memory. For example, non-monolithic stacked memories can beconstructed by forming memory levels on separate substrates and thenstacking the memory levels atop each other. The substrates may bethinned or removed from the memory device levels before stacking, but asthe memory device levels are initially formed over separate substrates,the resulting memory arrays are not monolithic three dimensional memoryarrays. Further, multiple two dimensional memory arrays or threedimensional memory arrays (monolithic or non-monolithic) may be formedon separate chips and then packaged together to form a stacked-chipmemory device.

Associated circuitry is typically required for operation of the memoryelements and for communication with the memory elements. As non-limitingexamples, memory devices may have circuitry used for controlling anddriving memory elements to accomplish functions such as programming andreading. This associated circuitry may be on the same substrate as thememory elements and/or on a separate substrate. For example, acontroller for memory read-write operations may be located on a separatecontroller chip and/or on the same substrate as the memory elements.

One of skill in the art will recognize that this invention is notlimited to the two dimensional and three dimensional exemplarystructures described but cover all relevant memory structures within thespirit and scope of the invention as described herein and as understoodby one of skill in the art.

Physical Memory Structure

FIG. 2 illustrates schematically a non-volatile memory cell. The memorycell 10 can be implemented by a field-effect transistor having a chargestorage unit 20, such as a floating gate or a charge trapping(dielectric) layer. The memory cell 10 also includes a source 14, adrain 16, and a control gate 30.

There are many commercially successful non-volatile solid-state memorydevices being used today. These memory devices may employ differenttypes of memory cells, each type having one or more charge storageelement.

Typical non-volatile memory cells include EEPROM and flash EEPROM.Examples of EEPROM cells and methods of manufacturing them are given inU.S. Pat. No. 5,595,924. Examples of flash EEPROM cells, their uses inmemory systems and methods of manufacturing them are given in U.S. Pat.Nos. 5,070,032, 5,095,344, 5,315,541, 5,343,063, 5,661,053, 5,313,421and 6,222,762. In particular, examples of memory devices with NAND cellstructures are described in U.S. Pat. Nos. 5,570,315, 5,903,495,6,046,935. Also, examples of memory devices utilizing dielectric storageelements have been described by Eitan et al., “NROM: A Novel LocalizedTrapping, 2-Bit Nonvolatile Memory Cell,” IEEE Electron Device Letters,vol. 21, no. 11, November 2000, pp. 543-545, and in U.S. Pat. Nos.5,768,192 and 6,011,725.

In practice, the memory state of a cell is usually read by sensing theconduction current across the source and drain electrodes of the cellwhen a reference voltage is applied to the control gate. Thus, for eachgiven charge on the floating gate of a cell, a corresponding conductioncurrent with respect to a fixed reference control gate voltage may bedetected. Similarly, the range of charge programmable onto the floatinggate defines a corresponding threshold voltage window or a correspondingconduction current window.

Alternatively, instead of detecting the conduction current among apartitioned current window, it is possible to set the threshold voltagefor a given memory state under test at the control gate and detect ifthe conduction current is lower or higher than a threshold current(cell-read reference current). In one implementation the detection ofthe conduction current relative to a threshold current is accomplishedby examining the rate the conduction current is discharging through thecapacitance of the bit line.

FIG. 3 illustrates the relation between the source-drain current I_(D)and the control gate voltage V_(CG) for four different charges Q1-Q4that the floating gate may be selectively storing at any one time. Withfixed drain voltage bias, the four solid I_(D) versus V_(CG) curvesrepresent four of seven possible charge levels that can be programmed ona floating gate of a memory cell, respectively corresponding to fourpossible memory states. As an example, the threshold voltage window of apopulation of cells may range from 0.5V to 3.5V. Seven possibleprogrammed memory states “0”, “1”, “2”, “3”, “4”, “5”, “6”, and anerased state (not shown) may be demarcated by partitioning the thresholdwindow into regions in intervals of 0.5V each. For example, if areference current, IREF of 2 .mu.A is used as shown, then the cellprogrammed with Q1 may be considered to be in a memory state “1” sinceits curve intersects with I_(REF) in the region of the threshold windowdemarcated by VCG=0.5V and 1.0V. Similarly, Q4 is in a memory state “5”.

As can be seen from the description above, the more states a memory cellis made to store, the more finely divided is its threshold window. Forexample, a memory device may have memory cells having a threshold windowthat ranges from −1.5V to 5V. This provides a maximum width of 6.5V. Ifthe memory cell is to store 16 states, each state may occupy from 200 mVto 300 mV in the threshold window. This will require higher precision inprogramming and reading operations in order to be able to achieve therequired resolution.

NAND Structure

FIG. 4 illustrates schematically a string of memory cells organized intoa NAND string. A NAND string 50 comprises a series of memory transistorsM1, M2, . . . Mn (e.g., n=4, 8, 16 or higher) daisy-chained by theirsources and drains. A pair of select transistors S1, S2 controls thememory transistor chain's connection to the external world via the NANDstring's source terminal 54 and drain terminal 56 respectively. In amemory array, when the source select transistor 51 is turned on, thesource terminal is coupled to a source line (see FIG. 5). Similarly,when the drain select transistor S2 is turned on, the drain terminal ofthe NAND string is coupled to a bit line of the memory array. Eachmemory transistor 10 in the chain acts as a memory cell. It has a chargestorage element 20 to store a given amount of charge so as to representan intended memory state. A control gate 30 of each memory transistorallows control over read and write operations. As will be seen in FIG.5, the control gates 30 of corresponding memory transistors of a row ofNAND string are all connected to the same word line. Similarly, acontrol gate 32 of each of the select transistors S1, S2 providescontrol access to the NAND string via its source terminal 54 and drainterminal 56 respectively. Likewise, the control gates 32 ofcorresponding select transistors of a row of NAND string are allconnected to the same select line.

When an addressed memory transistor 10 within a NAND string is read oris verified during programming, its control gate 30 is supplied with anappropriate voltage. At the same time, the rest of the non-addressedmemory transistors in the NAND string 50 are fully turned on byapplication of sufficient voltage on their control gates. In this way, aconductive path is effectively created from the source of the individualmemory transistor to the source terminal 54 of the NAND string andlikewise for the drain of the individual memory transistor to the drainterminal 56 of the cell. Memory devices with such NAND string structuresare described in U.S. Pat. Nos. 5,570,315, 5,903,495, 6,046,935.

FIG. 5 illustrates an example of a NAND array 210 of memory cells,constituted from NAND strings 50 such as that shown in FIG. 4. Alongeach column of NAND strings, a bit line such as bit line 36 is coupledto the drain terminal 56 of each NAND string. Along each bank of NANDstrings, a source line such as source line 34 is coupled to the sourceterminals 54 of each NAND string. Also the control gates along a row ofmemory cells in a bank of NAND strings are connected to a word line suchas word line 42. The control gates along a row of select transistors ina bank of NAND strings are connected to a select line such as selectline 44. An entire row of memory cells in a bank of NAND strings can beaddressed by appropriate voltages on the word lines and select lines ofthe bank of NAND strings.

FIG. 6 illustrates a page of memory cells, organized in the NANDconfiguration, being sensed or programmed in parallel. FIG. 6essentially shows a bank of NAND strings 50 in the memory array 210 ofFIG. 5, where the detail of each NAND string is shown explicitly as inFIG. 4. A physical page, such as the page 60, is a group of memory cellsenabled to be sensed or programmed in parallel. This is accomplished bya corresponding page of sense amplifiers 212. The sensed results arelatched in a corresponding set of latches 214. Each sense amplifier canbe coupled to a NAND string via a bit line. The page is enabled by thecontrol gates of the cells of the page connected in common to a wordline 42 and each cell accessible by a sense amplifier accessible via abit line 36. As an example, when respectively sensing or programming thepage of cells 60, a sensing voltage or a programming voltage isrespectively applied to the common word line WL3 together withappropriate voltages on the bit lines.

Physical Organization of the Memory

One important difference between flash memory and other of types ofmemory is that a cell must be programmed from the erased state. That isthe floating gate must first be emptied of charge. Programming then addsa desired amount of charge back to the floating gate. It does notsupport removing a portion of the charge from the floating gate to gofrom a more programmed state to a lesser one. This means that updateddata cannot overwrite existing data and must be written to a previousunwritten location.

Furthermore erasing is to empty all the charges from the floating gateand generally takes appreciable time. For that reason, it will becumbersome and very slow to erase cell by cell or even page by page. Inpractice, the array of memory cells is divided into a large number ofblocks of memory cells. As is common for flash EEPROM systems, the blockis the unit of erase. That is, each block contains the minimum number ofmemory cells that are erased together. While aggregating a large numberof cells in a block to be erased in parallel will improve eraseperformance, a large size block also entails dealing with a largernumber of update and obsolete data.

Each block is typically divided into a number of physical pages. Alogical page is a unit of programming or reading that contains a numberof bits equal to the number of cells in a physical page. In a memorythat stores one bit per cell, one physical page stores one logical pageof data. In memories that store two bits per cell, a physical pagestores two logical pages. The number of logical pages stored in aphysical page thus reflects the number of bits stored per cell. In oneembodiment, the individual pages may be divided into segments and thesegments may contain the fewest number of cells that are written at onetime as a basic programming operation. One or more logical pages of dataare typically stored in one row of memory cells. A page can store one ormore sectors. A sector includes user data and overhead data.

All-Bit, Full-Sequence MLC Programming

FIGS. 7A-7C illustrate an example of programming a population of 4-statememory cells. FIG. 7A illustrates the population of memory cellsprogrammable into four distinct distributions of threshold voltagesrespectively representing memory states “0”, “1”, “2” and “3”. FIG. 7Billustrates the initial distribution of “erased” threshold voltages foran erased memory. FIG. 7C illustrates an example of the memory aftermany of the memory cells have been programmed. Essentially, a cellinitially has an “erased” threshold voltage and programming will move itto a higher value into one of the three zones demarcated by verifylevels vV₁, vV₂ and vV₃. In this way, each memory cell can be programmedto one of the three programmed states “1”, “2” and “3” or remainun-programmed in the “erased” state. As the memory gets moreprogramming, the initial distribution of the “erased” state as shown inFIG. 7B will become narrower and the erased state is represented by the“0” state.

A 2-bit code having a lower bit and an upper bit can be used torepresent each of the four memory states. For example, the “0”, “1”, “2”and “3” states are respectively represented by “11”, “01”, “00” and‘10”. The 2-bit data may be read from the memory by sensing in“full-sequence” mode where the two bits are sensed together by sensingrelative to the read demarcation threshold values rV₁, rV₂ and rV₃ inthree sub-passes respectively.

3-D NAND Structures

An alternative arrangement to a conventional two-dimensional (2-D) NANDarray is a three-dimensional (3-D) array. In contrast to 2-D NANDarrays, which are formed along a planar surface of a semiconductorwafer, 3-D arrays extend up from the wafer surface and generally includestacks, or columns, of memory cells extending upwards. Various 3-Darrangements are possible. In one arrangement a NAND string is formedvertically with one end (e.g. source) at the wafer surface and the otherend (e.g. drain) on top. In another arrangement a NAND string is formedin a U-shape so that both ends of the NAND string are accessible on top,thus facilitating connections between such strings. Examples of suchNAND strings and their formation are described in U.S. PatentPublication Number 2012/0220088 and in U.S. Patent Publication Number2013/0107628, which are hereby incorporated by reference in theirentirety.

FIG. 8 shows a first example of a NAND string 701 that extends in avertical direction, i.e. extending in the z-direction, perpendicular tothe x-y plane of the substrate. Memory cells are formed where a verticalbit line (local bit line) 703 passes through a word line (e.g. WL0, WL1,etc.). A charge trapping layer between the local bit line and the wordline stores charge, which affects the threshold voltage of thetransistor formed by the word line (gate) coupled to the vertical bitline (channel) that it encircles. Such memory cells may be formed byforming stacks of word lines and then etching memory holes where memorycells are to be formed. Memory holes are then lined with a chargetrapping layer and filled with a suitable local bit line/channelmaterial (with suitable dielectric layers for isolation).

As with planar NAND strings, select gates 705, 707, are located ateither end of the string to allow the NAND string to be selectivelyconnected to, or isolated from, external elements 709, 711. Suchexternal elements are generally conductive lines such as common sourcelines or bit lines that serve large numbers of NAND strings. VerticalNAND strings may be operated in a similar manner to planar NAND stringsand both SLC and MLC operation is possible. While FIG. 8 shows anexample of a NAND string that has 32 cells (0-31) connected in series,the number of cells in a NAND string may be any suitable number. Not allcells are shown for clarity. It will be understood that additional cellsare formed where word lines 3-29 (not shown) intersect the localvertical bit line.

A 3D NAND array can, loosely speaking, be formed tilting up therespective structures 50 and 210 of FIGS. 5 and 6 to be perpendicular tothe x-y plane. In this example, each y-z plane corresponds to the pagestructure of FIG. 6, with m such plane at differing x locations. The(global) bit lines, BL1-m, each run across the top to an associatedsense amp SA1-m. The word lines, WL1-n, and source and select linesSSL1-n and DSL1-n, then run in x direction, with the NAND stringconnected at bottom to a common source line CSL.

FIGS. 9-12 look at a particular monolithic three dimensional (3D) memoryarray of the NAND type (more specifically of the “BiCS” type), where oneor more memory device levels are formed above a single substrate, inmore detail. FIG. 9 is an oblique projection of part of such astructure, showing a portion corresponding to two of the page structuresin FIG. 5, where, depending on the embodiment, each of these couldcorrespond to a separate block or be different “fingers” of the sameblock. Here, instead to the NAND strings lying in a common y-z plane,they are squashed together in the y direction, so that the NAND stringsare somewhat staggered in the x direction. On the top, the NAND stringsare connected along global bit lines (BL) spanning multiple suchsub-divisions of the array that run in the x direction. Here, globalcommon source lines (SL) also run across multiple such structures in thex direction and are connect to the sources at the bottoms of the NANDstring, which are connected by a local interconnect (LI) that serves asthe local common source line of the individual finger. Depending on theembodiment, the global source lines can span the whole, or just aportion, of the array structure. Rather than use the local interconnect(LI), variations can include the NAND string being formed in a U typestructure, where part of the string itself runs back up, such as isdescribed in U.S. patent application Ser. No. 13/927,659, filed on Jun.26, 2013.

To the right of FIG. 9 is a representation of the elements of one of thevertical NAND strings from the structure to the left. Multiple memorycells are connected through a drain select gate SGD to the associatedbit line BL at the top and connected through the associated sourceselect gate SDS to the associated local source line LI to a globalsource line SL. It is often useful to have a select gate with a greaterlength than that of memory cells, where this can alternately be achievedby having several select gates in series (as described in U.S. patentapplication Ser. No. 13/925,662, filed on Jun. 24, 2013), making formore uniform processing of layers. Additionally, the select gates areprogrammable to have their threshold levels adjusted, aspects of whichare described in US patent publication number 2014-0169095. Thisexemplary embodiment also includes several dummy cells at the ends thatare not used to store user data, as their proximity to the select gatesmakes them more prone to disturbs.

FIG. 10 shows a top view of the structure for two blocks in theexemplary embodiment. Two blocks (BLK0 above, BLK1 below) are shown,each having four fingers that run left to right. The word lines andselect gate lines of each level also run left to right, with the wordlines of the different fingers of the same block being commonlyconnected at a “terrace” and then on to receive their various voltagelevel through the word line select gates at WLTr. The word lines of agiven layer in a block can also be commonly connected on the far sidefrom the terrace. The selected gate lines can be individual for eachlevel, rather common, allowing the fingers to be individually selected.The bit lines are shown running up and down the page and connect on tothe sense amp circuits, where, depending on the embodiment, each senseamp can correspond to a single bit line or be multiplexed to several bitlines.

FIG. 11 shows a side view of one block, again with four fingers. In thisexemplary embodiment, the select gates SGD and SGS at either end of theNAND strings are formed of four layers, with the word lines WLin-between, all formed over a CPWELL. A given finger is selected bysetting its select gates to a level VSG and the word lines are biasedaccording to the operation, such as a read voltage (VCGRV) for theselected word lines and the read-pass voltage (VREAD) for thenon-selected word lines. The non-selected fingers can then be cut off bysetting their select gates accordingly.

FIG. 12 illustrates some detail of an individual cell. A dielectric coreruns in the vertical direction and is surrounded by a channel siliconlayer, that is in turn surrounded a tunnel dielectric (TNL) and then thecharge trapping dielectric layer (CTL). The gate of the cell is hereframed of tungsten with which is surrounded by a metal barrier and isseparated from the charge trapping layer by blocking (BLK) oxide and ahigh K layer.

Fast Programming of Select Gates Suitable for BiCS (3D) MemoryTechnology

As noted in the preceding section, the sort of memory structuredescribed with respect to FIGS. 9-12 present a memory structure having aNAND type of architecture in which the select gates are programmable.This section considers techniques for programming these select gates.The select gates differ from the memory cells, among other ways, in thatthey are directly connected to either the bit line on the one end of theNAND string or to the source line on the other, whereas the memory cellshave these select gates separating them from these lines to be used tocontrol the bias levels to the memory cells. This makes accuratelysetting the threshold voltage (Vt) of the select gates trickier that theusual programming of a strings memory cells. More obviously, the selectgates differ in that they have their Vt programmed in order allow themto properly perform their function, as opposed to storing data values.As this sort of structure arises in the BiCS type structure described inthe last section, the following is presented in that context, althoughit is more generally applicable to other NAND type architectures havingselect gates with tunable threshold levels.

The need for properly setting the select gates' threshold voltages canbe illustrated with respect to FIGS. 13, 14A, and 14B. FIG. 13 shows atypical set of bias waveforms for programming selected memory cells in aNAND type memory. The source (SGS) and drain (SGD) side select gates areset low to ground (VSS), as are the bit lines (BL) corresponding tocells to be programmed, while program disabled bit lines are set to aVBL level (2V in this example) and the source line is set at VSL (here1.5V). The non-selected word lines are then taken to VPASS and theselected word lines up to the full program voltage of VPGM. The result,for a NAND string with properly tuned selected gates having a positivethreshold voltage is shown at FIG. 14A: the select gates are off, thechannel is boosted, and programming occurs. If, instead, the selectswere to have a negative threshold voltage (SG Vt<0V) as illustrated inFIG. 14B, they will conduct even though their gates are at 0V, and theselected memory cells will not program properly.

Negative select gate threshold voltages can also cause problems whenreading. FIG. 15 shows an exemplary set of bias voltages for selectedand unselected word line, source and drain select gates, bit lines andsource lines for a selected string in a selected block (top), anunselected string in a selected block (middle), and unselected stringfrom unselected blocks (bottom). For selected strings, both source anddrain selects should be on (at VSG) so the selected cells can be read;but for unselected strings in both selected and unselected blocks, theselect gates are set biased at VSS so that they are turned off and willnot contributed to the result of the sensing operation. However, if anyof the select gates in the unselected string (in either selected orunselected blocks) have negative thresholds, they may be on to somedegree and affect the result of the read operation and lead toinaccurate sensing. FIG. 16 illustrates the mechanism in the BiCScontext: here, the string at the left of the left most finger isselected, with any current flowing off to the source line CELSR, asindicated by the arrow. However, if the strings along the same bit lineof the other fingers are not fully cut off by their select gates, theymay also have some current flowing, as indicated at the lighter arrows,affecting the sensing result.

FIG. 17 is a two dimensional projection of a block of a typical BiCStype structure, showing one NAND string sharing a common bit line foreach of four finger, to illustrate some of the structure on how theselect gates and memory cells are arranged. Each finger has itsindividually controllable set of drain select gates, SCG0-3, and sourceselect gates, SGS0-3, along with the common source gate. The word linesWL0-n then run across all four fingers. The select gates can then beprogrammed similarly to the process for memory cells, but with somedifferences. As with the memory cells, programming pulses are applied tothe control gates (now by way of the select lines, rather than wordlines) with intervening verify operations, but where these are acollective verify for the commonly controlled sets of select gates; thatis, the four select gates all controlled by SGD0 would be verified as aunit, where they are being programmed to tune their threshold voltage asopposed to storing a data value.

A difficulty in programming these sets of select gates is that, unlikethe memory cells along the word lines, they are not separated from thebit line BL by a separate select gate on the drain end nor are theyseparated from the source line on the other end. This lack of a “selectgate for the select gates” make it difficult to set the proper biaslevels allowing the select gates to be programmed in the same way as thememory cells. For instance, on the drain side this means that the bitline needs to be biased to higher level that is typically provided bythe sense amp in order to inhibit programming, requiring an elevatedlevel from a charge pump. And the common SGC line and CELSRC level doesnot allow for inhibiting the source side selected transistors ofindividual fingers. Also, due the amount of coupling due to the densityof the BiCS structure, it is often difficult to program along all of thebit lines, whether for word lines or select gates together, so that onlyevery other or even every fourth bit line is done concurrently.

To overcome these limitations, the exemplary embodiment uses a modifiedstructure for the select gates. On the drain side, the end select gateconnected to the bit line is separately controllable from the otherdrain side select gates, allowing to be used more typically like a drainside select gate during the programming of the other drain side selectgates, which can still be commonly controlled. On the source side, theend select gate connected to the local common source line is alsocontrollable separately from the other source side select gates,allowing it to be biased separately for programming of the other sourceselect gates. This structure is illustrated in FIG. 18, which can becompared to FIG. 17. The levels shown are for programming the threedrain select transistors on the right most NAND string.

In FIG. 18 four NAND strings, one each from four fingers of a block, areshown. On the drain side, the three select gates adjacent to the wordlines in each finger again have commonly connected control gates. Theend most drain select gate, through which the finger connects to the bitline BL, is controlled separately. In the exemplary embodiment the endselect gates of the four fingers are all connected along the samecontrol line SGD4. A similar structure is used along the bottom, withthe bottom transistors of the four fingers, through which each NANDstrings connects to the local source line, commonly connected alongcontrol line SGS4, which also controls the transistors between the NANDstrings and the source line inter-connect columns. The exemplaryembodiments here, such as that illustrated in FIG. 18, have four selectgates on either end of each NAND string, where the end-most of each setis controlled independently of the other three, which are commonlycontrolled, but the techniques of the is section are more generallyapplicable. For example, on either end, one or more end-most selecttransistors can serve a more or less standard select gate function whileprogramming of one or more additional select transistors with adjustablethresholds to the inside, where this can be implemented independently oneither the source or drain sides. Consequently, the threshold tuning ofselect gates' threshold becomes similar to the programming of memorycells, but where (when there is more than one of such adjustable Vtselect gates) the verify is a combined verify and rather than beingwritten to a data state they are having there thresholds set to awell-defined level to allow proper operation of the NAND string.

When the right most finger of FIG. 18 is selected for programming of thethreshold on the source side select gates, all of the word lines aretreated as unselected and set at the programming pass level VPASS(typically on the order of 10V), as are the levels SGD0-2 on the drainside select gates of non-selected fingers. The source side select gatelevel SGS0-3 are set to VSGS, a level sufficient to turn them on. Theend source side select gate line SGS4 is set to VSS (0V) to shut off theNAND string on the source end and also the gates between local sourceline interconnects. The end most drain side select gate control lineSGD4 is then set at VSGD, a value so that it either conducts or is offbased on whether the bit line is program inhibited or program enabled;for example, something like 4V. To enable programming the bit line BL isset at VSS and to program inhibit the bit line is raised to a high levelfrom the sense amp, here a level VDDSA used for pre-charging bit linesthat can be in the 2.5V-4V range. (More information on sense amps andpre-charge levels can be found in US patent publication number2014-0003157.) On the source line, when programming CELSRC is set high(such as VDDSA) to shut off the transistors, and once verified or ifnon-selected CELSRC can be set at the threshold voltage of a depleteddevice, Vtd (.about.1V), to turn on the transistors.

To adjust the selected drain select gates threshold voltages byprogramming them up, the bit line is set to the program enable level(VSS). This establishes the sort of situation that FIG. 19 illustratesschematically, where the PASS voltage is on the word lines (the end mostof which is shown) and the programming voltage is on the selected drainselect gates, boosting the channel to a level VBOOST, and the bit lineis either at VSS to enable or, as shown, VDDSA to inhibit programming.Once biased, at set of programming pulses of VPGM is applied on thecommonly controlled to the select transistors, with alternating verifyoperations between the pulses. Once the select gates verify as beingtuned to the desired range, they are then locked out from furtherprogramming. In both the pulse and verify operations, as the controlgates of the select transistors are commonly connected, the verify is acombined verify to determine if the set of transistors have, as a group,the desired collective threshold value for effective use as a singleselect transistor.

Under this arrangement, the VBOOST does not need to be particularlylarge in SGD programming (.about.3 v to 4 v range) and a largelyconventional programming process can be used. Under this arrangement, anall bit line program can be done for the select gates to improveperformance, whereas for the memory cells only every other or everyfourth bit line are programmed concurrently in the BiCS type structuredue to the amount of capacitive coupling the results from high densitystructure. By separating out the end most select gate, the others can beprogrammed without need of a charge pump to pre-charge bit lines to ahigher than standard level, saving on power.

FIG. 20 is similar to FIG. 18, but for programming of the source sideselect gates. The biasing is largely as before, except that now on theselected (right most) NAND string, the commonly connected drain sideselect gates are also now at VPASS to provide channel boosting. This isillustrated in FIG. 21, that is source side equivalent of FIG. 19. Thecommonly connected gates of the source select gates then receive theprogramming pulses, alternated with a common verify operation. This canprovide the same advantages as for the drain side process in terms ofspeed, power and accuracy, allowing for the individual programming ofeach finger's source side select gates.

CONCLUSION

The foregoing detailed description of the invention has been presentedfor purposes of illustration and description. It is not intended to beexhaustive or to limit the invention to the precise form disclosed. Manymodifications and variations are possible in light of the aboveteaching. The described embodiments were chosen in order to best explainthe principles of the invention and its practical application, tothereby enable others skilled in the art to best utilize the inventionin various embodiments and with various modifications as are suited tothe particular use contemplated. It is intended that the scope of theinvention be defined by the claims appended hereto.

What is claimed is:
 1. A method of programming non-volatile memory,comprising: applying a program enable voltage to a bit line; applying apass voltage to a control gate of each of a plurality of non-volatilememory cells coupled to the bit line through a plurality ofserially-connected drain select transistors including a first drainselect transistor and two or more second drain select transistors; andapplying the pass voltage to a control gate of the first drain selecttransistor while applying a program voltage to a control gate of each ofthe second drain select transistors to concurrently program the seconddrain select transistors independently of the first drain selecttransistor.
 2. The method of claim 1, wherein: applying the pass to thecontrol gate of the first drain select transistor comprises applying aconducting voltage level to a first control line coupled to the controlgate of the first drain select transistor; and applying the programvoltage to the control gate of each of the second drain selecttransistors comprises applying the program voltage to a second controlline coupled to the control gate of each of the second drain selecttransistors.
 3. The method of claim 2, wherein: the second drain selecttransistors have programmable threshold voltages.
 4. The method of claim3, wherein: the first drain select transistor is coupled to the bit lineand the two or more second drain select transistors are coupled to thebit line via the first drain select transistor.
 5. The method of claim3, wherein: the threshold voltages of the second drain selecttransistors are jointly programmable; and the first drain selecttransistor is directly coupled to the bit line without an interveningtransistor.
 6. The method of claim 3, wherein the threshold voltages ofthe second drain select transistors are configured to have positivevalues.
 7. The method of claim 2, further comprising: programming theplurality of non-volatile memory cells by biasing the second controlline at a voltage level below the threshold voltages of the second drainselect transistors.
 8. The method of claim 1, further comprising:turning off a first source select transistor coupled to the plurality ofnon-volatile memory cells while applying the program enable level to thebit line.
 9. The method of claim 8, wherein: the first source selecttransistor is coupled to a source line and a second source selecttransistor is coupled to the source line via the first source selecttransistor; a control gate of the first source select transistor iscontrollable separately from a control gate of the second source selecttransistor; and the method further comprises turning on the secondsource select transistor while concurrently programming the second drainselect transistors.
 10. The method of claim 2, wherein the plurality ofserially-connected drain select transistors is a first plurality ofserially-connected drain select transistors, the plurality ofnon-volatile memory cells is a first plurality of non-volatile memorycells, and the bit line is a first bit line, the non-volatile memoryfurther comprises: a second plurality of serially-connected drain selecttransistors each having a control gate; and a second plurality ofnon-volatile memory cells coupled to a second bit line through thesecond plurality of serially-connected drain select transistors, whereinthe second plurality of serially-connected drain select transistorscomprises a third drain select transistor coupled to the second bit lineand two or more fourth drain select transistors coupled to the secondbit line via the third drain select transistor, wherein the control gateof the third drain select transistor is coupled to the first controlline and the control gates of the fourth drain select transistors arecoupled to a third control line and wherein the method further comprisesconcurrently programming the fourth drain select transistorsindependently of the third drain select transistor.
 11. A method ofprogramming non-volatile memory, comprising: providing a first drainselect transistor coupled to a bit line and two or more second drainselect transistors coupled to the bit line via the first drain selecttransistor, the first drain select transistor is coupled to a firstcontrol line and the second drain select transistors are coupled to asecond control line, the second drain select transistors havingprogrammable threshold voltages; providing a plurality of memorytransistors coupled to the first and second drain select transistors;providing one or more source select transistors coupled to the pluralityof memory transistors; and concurrently programming the two or moresecond drain select transistors independently of the first drain selecttransistor.
 12. The method of claim 11, wherein: each second drainselect transistor has a programmable threshold voltage; and concurrentlyprogramming the two or more second drain select transistors comprisesjointly programming the threshold voltages of the two or more seconddrain select transistors.
 13. The method of claim 12, whereinconcurrently programming the two or more second drain select transistorscomprises: applying a conducting voltage level to the first controlline; and applying a program voltage to the second control line.
 14. Themethod of claim 13, wherein: concurrently programming the two or moresecond drain select transistors comprises programming the thresholdvoltages of the two or more second drain select transistorsindependently from a threshold voltage of the first drain selecttransistor.
 15. The method of claim 11, wherein concurrently programmingthe two or more second drain select transistors comprises: setting thebit line to a program enable level; setting a source line coupled to theone or more source select transistors to a high level; setting a controlgate of each of the memory transistors to a pass level; and turning offthe one or more source select transistors.
 16. The method of claim 15,further comprising: commonly verifying the two or more second drainselect transistors subsequent to concurrently programming; setting thebit line to a program inhibit level in response to successfullyverifying the two or more second drain select transistors; and settingthe source line to a low level in response to successfully verifying thetwo or more second drain select transistors.
 17. The method of claim 16,wherein: each of the second drain select transistors comprises a controlgate; and the method further comprises setting the control gate of thefirst drain select transistor at a level to be turned on when the bitline is at the program enable level and turned off when the bit line isat the program inhibit level.
 18. A method of programming non-volatilememory, comprising: applying a program enable level to a bit line;applying a pass voltage to a control gate of each of a plurality ofnon-volatile memory cells coupled to a source line through a pluralityof serially-connected source select transistors including a first sourceselect transistor and two or more second source select transistors; andconcurrently programming the second source select transistorsindependently of the first source select transistor by turning off thefirst source select transistor while applying a program voltage to acontrol gate of each of the second source select transistors.
 19. Themethod of claim 18, further comprising: applying the pass voltage to aplurality of serially connected drain select transistors coupled to thebit line while applying the program voltage.
 20. The method of claim 19,wherein concurrently programming the two or more second source selecttransistors comprises: setting a source line coupled to the plurality ofserially-connected source select transistors to a high level; turningoff the first source select transistor by grounding a first control linecoupled to the first source select transistor; and applying the programvoltage to a second control line coupled to the control gates of thesecond source select transistors.